If I know the source of a photo, I put it in the description (e.g. "From the collection of John Doe, grandson of subject"). I don't try to do this with photos brought over from other trees (unless such attribution is already in the description) for the very reason you note here - so many of the photos were pulled from a tree into FTM, then uploaded as part of a new public member tree, that in many cases the tree you pull it from is several hops removed from the original uploader.
I do also make sure that all the original pictures I upload to my tree have proper attribution (some are scanned from my own photo albums, or those of family members, or have been emailed to me by a family member).
I do also make sure that all the original pictures I upload to my tree have proper attribution (some are scanned from my own photo albums, or those of family members, or have been emailed to me by a family member).